1. Field of the Invention
The present invention relates to an image generating method and the like for rendering a three-dimensional object as seen from a given viewpoint by generating an image of the three-dimensional object and writing color information of the generated image into a rendering buffer.
2. Description of Related Art
Many recent techniques of computer graphics (hereinafter referred to as “CG” suitably) are for obtaining more realistic images, but various researches for obtaining pictorial images (hereinafter referred to “non-photorealistic rendering images (NPR images)” suitably) have been performed.
Technology development for obtaining especially cell animation style images among the pictorial images is vigorously performed. The cell animation style images are images having little gradation called as the so-called solid coating. However, full of expressions of characters can be represented by means of contours rimmed on monotonous coloring. As technologies for obtaining the cell animation style images, a technology for determining cell animation style coloring by utilizing the rendering processing of three-dimensional CG (for example, as disclosed in Japanese Patent Publication No. 3231029 corresponding to the U.S. Pat. No. 6,549,202), and a technology for generating contours (for example, as disclosed in Japanese Patent Publication No. 3253020) are known.
On the other hand, technology researches for generating handwriting style images such as oil paintings as pictorial images which are not the ones of the cell animation style, are being performed. As a technology for converting a photographic image taken on the spot to a handwriting style image, a technology for determining a shape of a touch (a brush image in the present application) by detecting edge intensity and an inclination of the edge by the use of an edge detection filter is known (for example, as disclosed in Japanese Patent Publication No. 2862080).
However, the cost (especially of time) related to image processing is enormous in the technology for generating a handwriting style image, and an effective technology for obtaining a handwriting style image for a shorter time is demanded accordingly. The technology disclosed in Japanese Patent Publication No. 2862080 also aims to obtain a handwriting style image for a shorter time. However, because the technology is for writing touches in edge directions detected by passing a photographic image taken on the spot through an edge detection filter, the technology can not always detect a contour accurately. Consequently, the phenomenon of the writing of touches in erroneous directions owing to the impossibleness of accurate extraction of a model part of the photographic image can occur.
In the three-dimensional CG, a three-dimensional object composed of a plurality of primitive surfaces (e.g. polygons) is arranged in an object space, and a two-dimensional image as seen from a given viewpoint is generated by performing the rendering of the object. Consequently, the contour parts or the like of the three-dimensional object can be specified in the coordinate system of the object space. Further, because the three-dimensional object is arranged in the object space in which the coordinate system is defined, the coordinates of a desired position can be accurately obtained by operation. Incidentally, these coordinate calculations cause a problem of being costly (especially in time). In particular, when an image is generated in real time (in the case where an image is generated at every frame) in a game machine or the like, the problem of costs (especially of time) is serious.